2. Slope
• Mathematicians have developed a useful
measure of the steepness of a line, called the
slope of the line.
• Slope compares the vertical change (the rise)
to the horizontal change (the run) when
moving from one fixed point to another along
the line
3. Slope is expressed by
1. An angle from horizontal i.e 24°45°
2. As a gradient i.e 1 in 4as a vertical fall
3. centimetre per meter
4. Gradient
• The method to calculate the Gradient is:
• Divide the change in height by the change in
horizontal distance
• Gradient = Change in Y/Change in X
•
5. Types of open pit slopes
• Slopes in quarries can result from a range of activities associated with the
working of the site. There are three principal types of slope to be
considered:
1. Excavated slopes (in which in situ ground is exposed);
2. Tipped slopes (where loose material is formed into mounds either
above ground in placed in existing voids); and
3. Natural slopes (which may be affected by excavations or placement of
loose materials).
On the basis of duration pit slopes can be
1. Short term slopes – stockpiles and soil mounds, intermediate quarry
faces;
2. Medium term slopes – soil mounds, quarry faces, screening bunds, etc.;
and
3. Long term slopes – quarry faces, spoil mounds, lagoon embankments,
screening bunds.
6. Types of open pit slopes
• Unimportant slope
• Average slope
• Critical slope
• Unimportant slope;- slope angles unimportant
economically and flat slopes can be used
• Average slope;- slopes angles are important but not
critical in determining economics of mining
• Critical slope;- slope angle critical in terms of both
economics of mining and safety of operations
7. Types of open pit slopes
• In open pit mining, mineral deposits are mined from
the ground surface and downward. Consequently,
pit slopes are formed as the ore is being extracted.
• The various types of pit slopes are
1. Bench slope
2. Overall pit slope
3. Current overall pit slope
4. Haul road slope,
5. Ramp slope
6. Inter ramp slope
7. dump slope
9. • Bench slope or bank angle: it the angle measured in degree between the
horizontal and an imaginary line connecting bench toe to the bench crest.
It is an inclined plane of the bench which limits it on the side worked out
space.
• overall pit slope angle: it the angle measured in degree between the
horizontal and an imaginary line connecting bottom bench toe to the top
bench crest.
• Current overall pit slope;- it the angle measured in degree between the
horizontal and an imaginary line connecting existing bottom bench toe to
the existing top bench crest at a given time.
• Ramp slope;-The gradient on a ramp is the grade line profile along the road
centre line, in the vertical plane.
• Dump slope;- it the angle measured in degree between the horizontal and
an imaginary line connecting toe of dump to the existing top crest at a
given time.
• The interramp slope angle is measured from toe to toe or crest to crest
exclusive of any ramps. By excluding ramps or other offsets the interramp
angle is kept static no matter how many benches are measured. The
interramp angle is similar to the face angle because it is static.
10. PIT SLOPE GEOMETRIES
• There are numbers of slope which inters into pit design. Care is needed so that there is no
confusion as to how they are calculated and what they mean . The main are bench slope, overall
pit slope , , ramp slope, interramp slope etc.
• There are three major component of a pit slope
1. Bench configuration
2. Interramp slope
3. Overall slope
• The bench configuration is defined by bench face angle, the bench height and bench width.
• The interramp angle is the slope angle produced by a number of benches. Where there are haul
roads ,working levels or other wide benches
• The overall pit slope is the angle of the line from the toe to the crest of the pit.and will be flatter
than interramp slope.
• It is important to consider all three components in slope design
• In order to make a quantities estimate of the stability of aslope analytical models amenable to
mathematical solutions must be used
• The requrements of these modeles are the failure geometry and assumption regarding materila
properties and stress distributions.
• While active mining is under way ,some working benches would be included in the overall slope.
• At the end of mining it is desired to leave the final pit slope as steep as possible, some of safety
benches will be reduced in width while others may be eliminated entirely.`
12. PIT SLOPE GEOMETRIES
• The primary components of a pit design are as follows:
1. Bench Geometry –
• The height of the benches is typically determined by the size of the shovel chosen
for the mining operation.
• The bench face angle is usually selected in such a way as to reduce, to an
acceptable level, the amount of material that will likely fall from the face or crest.
• The bench width is sized to prevent small wedges and blocks from the bench
faces falling down the slope and potentially impacting men and equipment.
• The bench geometry that results from the bench face angle and bench width will
ultimately dictate the inter-ramp slope angle. Double or triple benches can be
used in certain circumstances to steepen inter-ramp slopes.
2.Inter-ramp Slope – The maximum inter-ramp slope angle is typically dictated by the
bench geometry. However, it is also necessary to evaluate the potential for multiple
bench scale instabilities due to large-scale structural features such as faults, shear
zones, bedding planes, foliation etc. In some cases, these persistent features may
completely control the achievable inter-ramp angles and the slope may have to be
flattened to account for their presence
3. Overall Slope – The overall slope angle that is achieved in a pit is typically flatter
than the maximum inter-ramp angle due to the inclusion of haulage ramps. Other
factors that may reduce the overall slope angles are things such as rock mass
strength, groundwater pressures, blasting vibration, stress conditions and mine
equipment requirements
13. Mechanism of pit slope failure
• Mechanism of slope failure; when driving force exceed the
resisting force
• Factor of safety; the ratio of resisting force to the driving force, if
FS≤1 the slope will fail, if FS >the slope is theoretically stable
14. shear stress is a stress state where the stress is parallel to the surface of the material,
as opposed to normal stress when the stress is vertical to the surface. Shear stress is
relevant to the motion of fluids upon surfaces, which result in the generation of shear
stress.
A shear load is a force that tends to produce a sliding failure on a material along a
plane that is parallel to the direction of the force.
Shear strength is a material's ability to resist forces that can cause the internal structure of
the material to slide against itself. Adhesives tend to have high shear strength.
In engineering, shear strength is the strength of a material or component against the type
of yield or structural failure where the material or component fails in shear.
Shear strength is a term used in soil mechanics to describe the magnitude of the shear
stress that a soil can sustain.
Strength ;-the ability to resist being moved or broken by a force
Stress ;-pressure or tension exerted on a material objec
15. • Before mining the horizontal stress flows horizontal and vertical stresses due to
weight downward and are in equilibrium state. when an excavation is made the
flows horizontal and vertical stresses disturbed and equilibrium state break.
• With the excavation of the pit , the pre existing horizontal stresses are forced to
flow beneath the pit bottom an and around the pit
• The vertical stresses are also reduced through the removal of the rock overlying the
final slopes. This means that the rock lying between the pit outline and theses flow
lines largely distressed
• As a result of stress removal cracks/joints can open with a subsequent reduction in
cohesive and friction forces restraining the rock in place
• Further more ground water can more easily flow through these zones reducing the
effective normal force on potential
Failure plane
• As the pit is deepened the extend
of this distressed zone increases and
The consequence of failure becomes
more severe .the presence of adverse
structure like fault, dykes, weak zones
Etc further reduce resisting force
As soon as driving force exceed the
resisting force slope failure takes place
16. • Slope stability problem is greatest problem faced by the
open pit mining industry.
• The scale of slope stability problem is divided in to two
types:
1. Gross stability problem: It refer to large volumes of
materials which come down the slopes due to large
rotational type of shear failure and it involves deeply
weathered rock and soil.
2. Local stability problem: This problem which refers to much
smaller volume of material and these type of failure effect
one or two benches at a time due to shear plane jointing,
slope erosion due to surface drainage.
17. TYPES OF ROCK SLOPE FAILURES
• Failure in Earth and Rock mass
1. Plane Failure
2. Wedge Failure
3. Circular Failure
4. Toppling Failure Rock fall Fail
Failure in Earth, rock fill and spoil dumps and Embankments
1. Circular
2. Non-circular
3. semi-infinite slope
4. Multiple block plane wedge
5. Log spiral (bearing capacity of foundations)
6. Flow slides and Mud flow
7. Cracking
8. Gulling
9. Erosion Slide or Slump Figure.
18.
19.
20. Plane failure
• Simple plane failure is the easiest form of rock slope failure to analyze. It occurs when a discontinuity
striking approximately parallel to the slope face and dipping at a lower angle intersects the slope face,
enabling the material above the discontinuity to slide.
• A rock slope undergoes this mode of failure when combinations of discontinuities in the rock mass form
blocks or wedges within the rock which are free to move. The pattern of the discontinuities may be
comprised of a single discontinuity or a pair of discontinuities that intersect each other, or a
combination of multiple discontinuities that are linked together to form a failure mode.
• A planar failure of rock slope occurs when a mass of rock in a slope slides down along a relatively planar
failure surface. The failure surfaces are usually structural discontinuities such as bedding planes, faults,
joints or the interface between bedrock and an overlying layer of weathered rock.
• Plane failure can occur on the bench scale ,interramp scale and Pit wall scale
• The favorable conditions of plane failure are as follows:
1.The dip direction of the planar discontinuity must be within ( ±20o) of the dip direction of the slope face
2. The dip of the planar discontinuity must be less than the dip of the slope face (Daylight)
3.The dip of the planar discontinuity must be greater than the angle of friction of
the surface
21. • In open pit mining, mineral deposits are mined
from the ground surface and downward.
Consequently, pit slopes are formed as the ore is
being extracted.
• It is seldom, not to say never, possible to
maintain stable vertical slopes or pit walls of
substantial height even in very hard and strong
rock.
• The pit slopes must thus be inclined at some
angle to prevent failure of the rock mass
22. Wedge Failure:
• Wedge failure can occur in rock masses with two or more sets of discontinuities
whose lines of intersection are approximately perpendicular to the strike of the slope and
dip toward the plane of the slope.
• Wedge failure of rock slope results when rock mass slides along two intersecting
discontinuities, both of which dip out of the cut slope at an oblique angle to the cut face,
thus forming a wedge-shaped block
• Wedge failure can occur in rock mass with two or more sets of discontinuities whose lines of
intersection are approximately perpendicular to the strike of the slope and dip towards the
plane of the slope. This mode of failure requires that the dip angle of at least one joint
intersect is greater than the friction angle of the joint surfaces and that the line of joint
intersection intersects the plane of the slope.
• The necessary structural conditions for this failure are summarized as follows:
1. The trend of the line of intersection must approximate the dip direction of the slope face.
2. The plunge of the line of intersection must be less than the dip of the slope face. The line
of intersection under this
condition is said to daylight on
the slope.
3. The plunge of the line of
intersection must be greater
than the angle of friction of the
surface
23. Circular Failure
• Circular failures are generally occur in weak rock or soil slopes. Failures of this type do not necessarily occur along a
purely circular arc, some form of curved failure surface is normally apparent
• This failure can occurs in soil slopes, the circular method occurs when the joint sets are not very well defined. When
the material of the spoil dump slopes are weak such as soil, heavily jointed or broken rock mass
• The conditions under which circular failure occurs are follows:
1. When the individual particles of soil or rock mass, comprising the slopes are
small as compared to the slope.
2. When the particles are not locked as a result of their shape and tend to behave as soil.
• Types of circular failure
Circular failure is classified in three types depending on the area that is affected by the failure
surface. They are:-
(a) Slope failure: In this type of failure, the arc of the rupture surface meets the slope above the toe of the slope. This
happens when the slope angle is very high and the soil close to the toe posses the high strength.
(b) Toe failure: In this type of failure, the arc of the rupture surface meets the slope at the toe.
(c) Base failure: In this type of failure, the arc of the failure passes below the toe and in to base of the slope. This happens
when the slope angle is low and the soil below the base is softer and more plastic than the soil above the base.
24. Toppling failure
• Toppling failures occur when columns of rock, formed by steeply dipping
discontinuities in the rock structure and it involves overturning or rotation of rock
layers
• Toppling failures occur when columns of rock, formed by steeply dipping
discontinuities in the rock rotates about an essentially fixed point at or near the
base of the slope followed by slippage between the layers
• . The centre of gravity of the column or slab must fall outside the dimension of its
base in toppling failure. Jointed rock mass closely spaced and steeply dipping
discontinuity sets that dip away from the slope surface are necessary
prerequisites for toppling failure. The removal of overburden and the confining
rock, as is the case in mining excavations, can result in a partial relief of the
constraining stresses within the rock structure, resulting in a toppling failure
25. FACTORS AFFECTING SLOPE FAILURE
• Slope failure are often caused by processes that increase shear stress or decrease the shear strength of soil or rock mass. Residual soil and
weathered bed rock can be weekend by pre-existing discontinuities such as fault, bedding surface, foliation, cleavages, sheared zone, elict
joints ,dikes and sills.
• Slope failure occurs when the downward movements of material due to gravity and shear stresses exceeds the shear strength.
• Therefore, factors that tend to increase the shear stresses or decrease the shear strength increase the chances of failure of a slope.
• factors that tend to increase the shear stresses
1.REMOVAL OF SUPPORT
A. Erosions
1. By streams and rivers,
2. By glacial
3. By action of wave of water bodies and ocean
4. By successive wetting and drying(e.g winds ,freezing)
B. Natural slope movements(e.g falls, slides, settlements)
C. Human activity
1. Cuts and excavation
2. Removal of retaining wall or sheet piles
3. Drawdown's of bodies of water (e.g lakes ,lagoons)
2. OVERLOADING
A. By natural causes
1. Weight of precipitation(e.g rains and snow)
2. Accumulation of material because of past slides
B. By human activity
1. Construction of fill
2. Building and other overload on the crest
3. Water leakage in culverts, water pipes and sewers
3. TRANSITORY EFFECT ( EARTH QUACKS)
4. REMOVAL OF UNDER LYING MATERIAL THAT PROVIDES SUPPORT
1. By rivers and sea
2. By weathering
3. By underground erosion due to seepage, solvent action
4. By human activity e.g mining or excavation
5. By loss of strength of underlying material
5. INCRESE IN LATERAL PRESSURE
1. By water in crack and fissures
2. By freezing of water in cracks
3. By expansion of clay
26. FACTORS AFFECTING SLOPE FAILURE
• factors that tend to decrease the shear strength increase
1. Factors inherent in the nature of the material
• Composition
• Structure
• Secondary or inherited structures
• Stratification
2. Changes caused by weathering and physicochemical activities
• Wetting and drying processes
• Hydration
• Removal of cementing material
3. Effect of pore pressure
4. Change in structure
• Stress release
• Structural degradation
27.
28. Prediction of slope failure
• Forecasting potential slope failure in open pit mines is integral to maintaining safety and mine productivity
29. 1.Bulging ground appears at the base of a slope or a retaining wall *
2. Water breaks through the ground surface or appears in a location near or at the
base of a slope *
3. Fences, retaining walls, utility poles, or trees tilt or move *
4. Cracks appear on the slope *
5. Water pipes break *
6. Cracks appear on the ground or in the foundation *
7. Structures on slopes moving away from their original position
8. Doors or windows start to stick or jam *
9. Sunken or down-dropped road beds
10. Slowly developing, widening cracks on the ground or paved areas such as streets
or driveways *
11. Land movement and small slides could indicate unstable condition of the slope
that may lead to bigger failures *
12. Outside walls, walks, or stairs begin pulling away from building.
30. Signs of immediate danger
• THE signs of slope failure can materialise days, weeks, or
months before failure actually occurs. However, there are
some unmistakeable signs that manifest moments before a
landslide occurs. These should never be ignored and
evacuative action needs to be taken. Simply put, a landslide
is coming and you need to get out of its way immediately.
• A sudden decrease in creek water levels although rain is
still falling or has just stopped.
• A faint rumbling sound that increases in volume which
indicates the landslide is coming closer.
• Unusual sounds, such as trees cracking or boulders
knocking together, which could indicate moving debris.
31. Short-term prediction of mass movement
• Springs, seeps, or saturated ground in areas that have not typically been wet
before.
• New cracks or unusual bulges in the ground, street pavements or sidewalks.
• Soil moving away from foundations.
• Ancillary structures such as decks and patios tilting and/or moving relative to the
main house.
• Tilting or cracking of concrete floors and foundations.
• Broken water lines and other underground utilities.
• Leaning telephone poles, trees, retaining walls or fences
• Offset fence lines.
• Sunken or down-dropped road beds.
• Rapid increase in creek water levels, possibly accompanied by increased turbidity
(soil content).
• Sudden decrease in creek water levels though rain is still falling or just recently
stopped.
• Sticking doors and windows, and visible open spaces indicating jambs and frames
out of plumb.
• A faint rumbling sound that increases in volume is noticeable as the landslide
nears.
• Unusual sounds, such as trees cracking or boulders knocking together, might
indicate moving debris.
32. Slope Movement Monitoring
• Slope movement monitoring, even in its simplest form, should be carried out in all
mining situations.
• Regular visual inspection for signs of tension cracking, rock fall activity, slope
raveling, bulging in the slope face or heaving at the toe of a slope can provide
advanced warning of potential instability.
1. Visual slope monitoring by routine walkover inspections by the Geotechnical
Engineer. The Engineer compares the last visit observations with the latest one and
records any deleterious slope stability changes that may have occurred. The
recording of any changes that occur on the open pit slope faces by production
personnel during the shift is another way of visual slope monitoring. To this end,
slope hazard awareness lessons are conducted twice a year for all pit workers from
supervisor to operator.
2. Slope monitoring using the Geodetic Monitoring System (Geomos) survey
technique.
3. A slope monitoring report is issued containing the results of the visual monitoring
and the Geomos monitoring systems. The report has an action list of critical slope
stability issues requiring attention with time lines and responsible individuals
indicated. The report is issued to production, management and technical officials.
Depending on the duration of mining and the heights of the proposed mine slopes, an
array of reflective survey prisms located near the pit crest provides a baseline of
slope displacements, from which potential changes in slope behaviour can be
assessed. In potentially unstable areas where tension cracks or slope
33. Hazard of slope failure
• Physical impact can be of three types
1. Direct;- are those consequences incurred by direct physical contact with land slide itself
2. Indirect ;-are change brought about properties and behaviour of natural system as a result of landslide activity
3. Acute ;- immediate , short lived
4. Chronic;- delayed, long period
• Hazard to human life and property
• Injury and loss of life
• Property damage
• Failure of communication system
• Social and economic disruption
• Loss of productive land
• Ecological impact
• Change in hydrology
• Change in ground profile (topography)and land use pattern
• Change in soil and rock structure
• Loss of scenic beauty
• Loss of production,
• extra stripping costs to remove failed material,
• DGMS may close the mine
34. Aim of slope stability:
1. To understand the development and form of natural and man made
slopes and the processes responsible for different features.
2. To assess the stability of slopes under short-term (often during
construction) and long-term conditions.
3. To assess the possibility of slope failure involving natural or existing
engineered slopes.
4. To analyze slope stability and to understand failure mechanisms and the
influence of environmental factors.
5. To enable the redesign of failed slopes and the planning and design of
preventive and remedial measures, where necessary.
6. To study the effect of seismic loadings on slopes and embankments.
7. Safe, properly designed, scientifically engineered slope.
8. Profitability of open cast mines.
9. Design engineer/ scientist
